Crosslinked nylon by irradiation of a graft copolymer of nylon with an ethylene/acrylate or modified copolymer

ABSTRACT

THE NYLON ARTICLE SUCH AS FILMS AND FIBERS, ARE EFFECTIVELY CROSS-LINKED BY IRRADIATION TO OBTAIN DIMENSIONAL STABILITY, MEMORY AND RESISTANCE TO HEAT AND SOLVENTS WITHOUT SIGNIFICANT POLYMER DEGRADATION BY EMPOLYING A GRAFT OF A NYLON POLYMER GROWN ON AN ETHYLENE-ACRYLATE COPOLYMER AS INITIATOR AND RESTRICTING THE DOSE OF IONIZING RADIATION TO A LOW DOSE OF ABOUT 5 TO 20 MEGARADS. HIGHER DOSES GIVE A HIGHLY CROSS-LINKED POLYMER WITH SIGNIFICANTLY LOWER DEGRADATION THAN WITH NYLON HOMOPOLYMERS. THE PREFERRED ETHYLENE COPOLYMER HAS A PORTION OF THE ACRYLATE CONVERTED TO AMIDE.

Aug. 1, 1972 GAL IANQ E'I'AL 3,681,216

COPOLYMER CROS SLINKED NYLON BY IRRADIATION OF A GRAFT OF NYLON WITHANETHYLENE/ACRYLAI'E OR MODIFIED COPOLYMER F1led June 13, 1969 UnitedStates Patent 3,681216 CROSSLINKED NYLON BY IRRADIATION OF A GRAFTCOPOLYMER OF NYLON WITH AN ETHYLENE/ACRYLATE OR MODIFIED C- POLYMERFrancis R. Galiano, Prairie Village, and Samuel P.

Thackaberry, Wayne E. Smith, and Raymond P. Anderson, Overland Park,Kans., assignors to Custom Resins, Inc., Henderson, Ky.

Filed June 13, 1969, Ser. No. 832,910 Int. Cl. B0lj 1/00; C08d 1/00 U.S.Cl. 204159.14 11 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THEINVENTION This invention relates to a method of improving nylon bygrafting the nylon to a polyethylene copolymer then crosslinking withionizing radiation.

Crosslinking of nylon homopolymer is known in U.S. 2,858,259. The graftcopolyymer of nylon onto ethyleneacrylate copolymer is known in U.S.3388186. The ethylene-acrylate copolymer with acrylate partiallyconverted to amide groups is known in U.S. 3,337,517. Radiation ofpolyethylene homopolymers blended with nylon is known in the BelgianPat. No. 564,824.

However, the teachings of the prior art have not yet solved the problemof economically crosslinking nylon. A major drawback tocommercialization of ionizing radiation-crosslinked nylon has been thecost of power required in such processes. It has been necessary to usedose levels above 30 megarads to obtain significantly high degrees ofcrosslinking (Majury & Pinner, Irradiation of Polycaprolactam, J. Appl.Chem., 8, p. 168, March 1958). However, this highly crosslinked materialwill become embrittled from the high radiation doses due to polymerdegradation (ibid. p. 169).

SUMMARY OF THE INVENTION Use of a polyolefin copolymer as initiator forcaprolactam polymerization provides a means of preparing nylon 6 whichcan be highly crosslinked at radiation dose levels significantly lowerthan those required to crosslnk nylon 6 homopolymers. See theaccompanying drawing. The graft copolymer of nylon 6 prepared by thismethod crosslinks at about 5 megarads, while nylon 6 homopolymercrosslinks at about 35 megarads. Thus, irradiation of nylon 6 grafted topolyolefin copolymer is an economical process for the production ofnylon 6 3,681,216 Patented Aug. 1, 1972 with decreased solubility,decreased plastic fl0w above 250 C., and higher heat resistance atelevated temperatures.

Controlled ionizing radiation of ethylene copolymer 5 initiated nylon atdoses below 10 megarads provides an economical means for preparing highviscosity resin which is diflicult to obtain by conventional methods,and for tailoring solution viscosity and other resin properties to adesired level. Thus, the graft copolymers of this invention can becrosslinked at doses which have been found economically feasible onother polymers.

Also, by means of this invention, the graft copolymer of nylon on apolyolefin copolymer, such as an ethyleneacrylate copolymer, is notdegraded nearly as much as the sirnilar nylon homopolymers at doses ashigh "as 50 megarads. Thus, the graft copolymer of nylon on polyolefincopolymers can be highly crosslinked with significantly lowerdegradation than nylon homopolymers. However, at the present time itappears economically feasible to use a dose of 5 to 35, preferably 5 to20 megarads, to keep costs lower due to electric power requirements.

The ethylene copolymer can have almost any molecular weight. Acrylatecopolymers and methacrylate copolymers which have been converted mainlyto the amide form, but may contain some acid, ester and salt groups arepreferred. Some acid groups should be present initially or formedin-situ.

This process may be extended to include the radiation crosslinking ofnylon 6 prepared with a variety of polyolefin copolymer initiators.Among the polyolefin copolymers which may be useful in this process arestyrenemaleic anhydride (SMA) copolymers and their hydrolyzed andammoniated analogs. Poly-a-olefin maleic anhydride (Polymac) copolymersand their hydrolyzed and ammoniated analogs, and ethylene-acrylic acidco polymers are also useful. This graft copolymer may also becrosslinked by other means such as conventional peroxide free radicalagents.

This ionizing radiation of nylon 6-ethylene copolymer graft copolymercan be used to impart memory to film or fiber made of such graftcopolymer. For example, film could be prepared, crosslinked byradiation, cold drawn, then shrunk around a material to be packaged withthe use of heat. The shrinking would be the result of the memoryimparted to the film by the radiation process. Another use, noteconomically or technically feasible by the prior art methods, would beto impart this same kind of memory to fibers of this graft copolymerbyradiating the fibers in a finished piece of wearing apparel so that itwould have memory to retain its original shape, i.e. another type ofwash and wear fabric.

By ionizing radiation is meant radiation with suficient energy to removean electron from a gas atom, forming an ion pair; this requires anenergy of about 32 electron volts (ev.) for each ion pair formed. Thisradiation has suflicient energy to non-selectively break chemical bonds;thus, in round numbers radiation with energy of 50 electron volts (ev.)and above is efrective for the process of this invention. The ionizingradiation of the process of this invention is generally classed in twotypes: high energy particle radiation, and ionizing electromag neticradiation. The efect produced by these two types of radiation issimilar, the essential requisite being that the incident particles orphotons have suificient energy to break chemical bonds and generate freeradicals.

The method and crosslinked graft copolymer of this invention can beachieved by irradiating with ionizing radiation at a dosage of about toabout 20 megarads. By nylon is meant any polycarbonamide formingpolymers such as the monoaminocarboxylic acids or their correspondinglactams, or mixtures of diamines and dicarboxylic acid. By nylon 6 ismeant polymerized caprolactam. The preformed copolymer which the nylonis grafted onto is a copolymer of ethylene containing from about 5 toabout 90% by weight of chemically combined ethylene. The graft copolymermay contain from about 1 to about 30% by weight of preformed copolymer,and preferably about 2 to about 20% by weight of preformed copolymer.

The preformed copolymer is a copolymer of ethylene copolymerized with amonomer having the structural formula. when polymerized where R is H 01CH R is --OR OH NHg, NHR and NR R or OM, where M is a metal or NH.; Rand R.; are lower alkyl groups, and lower alkyl means alkyl groupscontainng one to eght carbon atoms. Preferably the preformed copolymercontains at least 2% by weight of the polymerized monomer units in thecopolymer where R is -NH -NHR or *N'RsR4.

DESCRIPTION OF THE DRAWING The drawing is a graph, showing percent gelvs. dose. The graft copolymer of this invention is shown in the curveson the left. The homopolymer curve on the right is taken from the Majury& Pinner publication cited in the background above.

PREFERRED EMBODIMENTS Preparation of ethylenemethyl acrylate copolymerAfter preliminary lighting the reactor using an ethylene feed andattaining reactor equilibrum, the desired reaction is commenced bygradually adverting to the required reaction feed. The reactants arecontinuously fed by pump ing into the fore part of reaction zone of ahigh pressure, agitated autoclave reactor at a ratio of 00067 mol ofmethyl acrylate per mol of ethylene and at a rate of about 500lb./hr./cu. ft. of reactor volume. The reaction feed employed has lowmoisture and oxygen contents. Lauroyl peroxide is injected as the freeradical initiator in an equivolume benzene mineral ol vehicle at a rateof 750 p.p.m. on the basis of the weight of feed. The reaction isconducted at a pressure of about 17,500 p.s.i. The temperature ismaintained in the range of 300 F. to about 430 F. for the most partcontrolled at around 360 F. The reactor is equipped with a longitudinalagitator which is rotated at a rate of about 250 r.p.m.

.The unconsumed reactants, along with the formed co polymer, aredischarged from the reactor in a conversion to the copolymer of from 15to 20% of the total weight of the reactants. Nearly all of theunpolymerised discharge of reactants constitutes ethylene. It isrecovered from the molten copolymer by means of separator vessels andcirculated for re-use.

The separated molten polymer is then extruded as a ribbon into a waterbath for coolng. The solidified copolymer of ethylene and methylacrylate is severed into cubes for convenient end use.

The copolymer has the following properties: a content of 0.054 mol ofmethyl acrylate/mol of ethylene of the polymer as deterrnined byinfrared absorption analysis, a melt index of 1.72, a density of 0.9365,and a Vicat softening point of 71 C.

Addtionally, the copolymer has an ultimate elongation value of 690percent. The copolymer shows an impact value of 2560 p.s.i., a yieldpoint of 492 p.s.i., and an ultmate tensle strength of 1380 p.s.i. Filmcast of the copolymer is highly elastomeric.

EXAMPLE I A quantity of 300 grams of the above described copolymerhaving a crystallinity of about 50.4%, is added to a 2-liter autoclaveammonolysis chamber along with 300 mls. of concentrated ammoniumhydroxide (15 N) and 600 mls. of xylene. The mixture is heated withconstant agitation at about 225 C. for about 12 hours. Upon completionof the reaction, the residue comprising the amide derivative of thepolymer is removed, washed with a methanol acetone mixture and is airdried. The dried amide polymer is ground, and a portion of the groundpolymer is pressed into film. The pressed film has a crystallinity of28% and is a relatively stiff and very tough film of high clarity.

EXAMPLE 11 A 250 g. quantity of an ethylenemethyl acrylate copolymercontaining 0.054 mole of methyl acrylate per mole of ethylene groups isemployed (the copolymer of Example I of British Pat. 900,969). Thestarting methyl acrylate copolymer has a density of 0.9365 g./cc., amelt index of 1.72, an ultimate tensile strength of 1380 p.s.i., anelongation value of 690 percent, an impact value of 2560 p.s.i., a yieldvalue of 492 p.s.i., an a flexural stiness value of 3000 lb./sq. in. Themethyl acrylate copolymer, together wtih 600 ml. of 17 N aqueousammonium hydroxide and 1400 m1. of p-xylene are added to a two-literpressure reactor. The reaction mixture is heated to 225 C. for 16.5hours with agitation by rocking. A pressure of about 850 p.s.i. isgenerated during the reaction. The reaction mixture is cooled and theamide polymer product which consists of a white solid is removed. Thepolymer product is washed with methanol and is ground to provide afinely divided product which is dried.

The amide polymer product has the following properties: 1.5 percentnitrogen; 50.6 percent of the starting ester groups are converted toamide groups; 198 ft. lbs./ sq. in. tensile impact; a melt index of 0.2;zero failures of ten flexes at 72 C. (brittleness value) and 13,500lbs./ sq. in. flexural stffness.

EXAMPLE IH Water, 425 ml. ammonia solution 28%, 1175 ml. and 350 g. ofan ethylene methyl methacrylate copolymer, containing 41% by weightmethyl methacrylate, and having a melt index of 1,000 were charged to aone gallon antoclave and heated at 260 C. for 6 hours, stirred at 1100r.p.m. Maximum pressure over the reaction pcriod was 1300 p.s.i. Polymersolids were collected as tiny spheres by vacuum filtration and washedfour times by slurrying with deionized water, followed by filtration.'Ihe polymer cake was dried at 45 C. and 28 inches of mercury. Thepolymer product contained about 60% by weight ethylene, about 32% byweight methacrylamide, and about 8% by weight methacrylic acid, and hada melt index of 90.

EXAMPLE IV Preparation of grafted polycarbonamide The followingprocedure illustrates a method for making suitable ethylene-acryliccopolymer-polycarbonamide grafts, employing a copolymer with a varietyof polymerzed monomer units, including about 2 weght percent acrylamdeunits.

the amount of material insoluble in cresol and formic acid increasing ineach instance as the dose is increased to 20 megarads.

'IABLE I.SOLUBILITY OF IRRADIA'IED NYLON 6 FILM Irradiation conditionsSolubility Dose, Environ- Formic Resin reference Initiator megarads mentacid M-cresol Commercial N lon Control Soluble...- Soluble.

y 5,10,1520 AiI........ ---do D0.

5,1020 N Soluble.... Soluble.

( Insoluble; Copolymer graited Nylon 6. 10% ethylene-acrylic eopoiymer:r..- 9095 Do.

60-70. Soluble. 50-60.

Temperature=7Z-90 F.

" Film solubility prior to irradiat n Solubility not determined. Greaterthan 95% soluble.

One and one-half parts, by weight, of a copolymer having chemicallycombined therein, by weight, 81.1 percent ethylene, 9.5 percent sodiumacrylate, 6.6 percent acrylic acid, 1.9 percent acrylamde and .9 percentmethyl acrylate and 150 parts of e-caprolactam were mixed in with 1.5parts of water in a resin kettle, and the mixture was heated slowly to100120 C. for 1 hour under a blanket of nitrogen. The temperature wasraised to 260-270 C. over one hour and the heating continued for anadditonal 9 hours. The product was a white, viscous material which wasallowed to cool to provide a resilient, hard resin, which was frezen inDry Ice, crushed and ground in a Wiley mill. The ground resin was thenextracted with methanol for 8 hours in a Soxhlet extractor, and aportion of the extracted product was injection molded into testspecimens characterized by a Shore D Hardness of 82, an average tensileimpact of 109.8 ft. lbs./infi a tensile yield of 11,568 lbs./sq. in. andbreak at 12,224 lbs./sq. in. elongation of 85 percent, a flexuralstiflness of 306,719 p.s.i. and a malt index at 235 C. of 6.3 gm./ 10minutes under a load of 2160 grams.

EXAMPLE V The polymer product of Example 111 was substituted for thecopolymer of Example IV and was used to prepare a graft copolymer by themethod of Example 1V with similar results.

Irradiation of grafted nylon Samples of copolymer-initiated nylon 6 werepressed into film at 450 F. (hydraulc press). The film samples wereirradiated with a linear eleciron accelerator at 72- 90 F. in air ornitrogen environment with high energy electrons up to dose levels ofmegarads. Dose rates used were 43,000 rads per second in air, and 54,000rads per Solubility of irradiated nylons As can be seen from the data inTable 1, commercial Nylon 6 does not become nsoluble in formic acid 01mcresol at doses up to 20 megarads. Samples of ethylene copolymergrafted Nylon 6 were rendered partially insoluble at dose levels between5 and 20 megarads, with Relatve solution viscosities of irradiatednylons Relative solution viscosities were determined for thoseirradiated samples of commercial Nylon 6 and the grafted nylon whichappeared to be soluble in formic acid. The data are summarized in Table'II.

At a dose of 20 megarads, the reiative solution viscosities of thecommercial nylon has increased somewhat over that obtained for controlsamples, but only to an extent normal for Nylon 6 homopolymer. Bycomparson, the relative solution viscosity of a sample of grafted nylonirradiated to only 5 megarads in nitrogen has shown a significantincrease over that shown by the control sample; the sample irradiated to5 megarads in air was not sufliciently soluble in formic acid to permitdetermination of the relative solution viscosity with accuracy.

" Film samples were dried at C. under vacuum prior to analysis. bRelative solution viseosity measured on film sample prior to irradia-Significant amount of material insoluble in formic acid. Very smallamount of material insoluble in orm.ic acid.

We claim:

1. Inadiatng with ionizing radiation at a dosage of from about 5 toabout 20 megarads an article formed from a composition comprising agraft copolymer of (A) from about 1 to about 30 weight percent of apreformed copolymer of ethylene and at least one comnomer said comonomerwhen p0lymerized having the structural formula R2 where R is H or CH Ris --0R -OH NH --NHR; and NR R or -OM where M is a metal or NH R and Rare lower alkyl groups, at least 2% by weight of the polymerzcd monomerunits in the copolymer have R as -NH NHR or -NR R with (B) from about 99to about 70 percent of polycarbonamide forming monomers selected fromthe group consisting of (a) monoaminomonocarboxylic acids or theircorresponding lactams (b) mixtures of diamines and dicarboxylc acids,and mixtures of (a) and (b), sad preformed copolymer conssting of fromabout to about 90% by weight of chemically combined ethylene.

2. The method of crosslinking nylon consistin-g essentially of preparinga graft copolymer of from about 99 to about 70 weight percent of a nylongrafted on to from about 1 to about 30 weght percent of a preformedcopolymer of ethylene copolymerized with a monomer having the structuralformula when polymerzed where R is -H or CH R is 0R OH --NH, NHR and NRR or -OM where M is a metal or NH R and R., are lower alkyl groups, atleast 2% by weight of the polymerized monomer units in the copolymerhave R as NH2, --NHR or -NR R sad preformed polymer consisting of fromabout 5 to about 90% of chemically combned ethylene,

irradiatng sad graft copolymer with electrons at a dosage of from about5 to about 35 megarads.

3. The method of claim 2 wherein the dosage is from about 5 to about 20megarads.

4. The method of claim 2 wherein the nylon is polycaprolactom.

5. The method of claim 3 wherein the graft copolymer is irradated at aclose of below megarads.

6. The method of claim 2 wherein the graft copolymer contains from about2 to about 20% by weight of: preformed copolymer.

7. A crosslinked graft copolymer of from about 99 to about 70 weightpercent of a nylon grafted on to from about 1 to about 30 weight percentof a preformed copolymer of ethylene copolymerized with a monomer haviugthe structural formula when polymerized where R is H 01 CH R: is 0R OH,-NH NHR and NR R 01 -OM where M is a metal or NH R and R a1e 1ower alkylgroups, at least 2% by weight of the polymerized monomer units in thecopolymer have R as NH2, NHR or NR R sad preformed polymer consisting offrom about 5 to about of chemically combined ethylene, and sad graftcopolymer has been crosslinked by rradiatin-g with electrons at a dosageof from about 5 to about 35 megarads.

8. The copolymer of claim 7 wherein the nylon is polycapraloctam.

9. The copolymer of claim 8 wherein sad graft copolymer has beencrosslinked at a dosage of below 10 megarads.

10. The copolymer of claim 7 wherein the graft copolymer contains fromabout 2 to about 20% by weight of preformed copolymer.

11. The copolymer of claim 7 wherein the dosage is from about 5 to about20 megarads.

References Cited UNITED STATES PATENTS 6/1970 Robb 260857 3/1968 R0bb26041 MURRAY TILLMAN, Primary Examiner R. B. TURER, Assistant Examiner

